I'm accessing a HATEOAS API provided by a 3rd party and for some reason the response we receive from them contains links with null values for href. This throws an exception. I can not change the response as I have no control over this API. Is there any way around this problem?
Below is an example of what the JSON looks like:
{
"_embedded": {
"example": [{ ... }]
}
"_links": {
"next": {
"href": null
},
"prev": {
"href": null
},
"self": {
"href": "https://bag.basisregistraties.overheid.nl/api/v1/panden"
}
}
}
When I make the request using the RestTemplate I get an IllegalArgumentException with the message "Template must not be null or empty!"
Caused by: java.lang.IllegalArgumentException: Template must not be null or empty!
at org.springframework.util.Assert.hasText(Assert.java:284)
at org.springframework.hateoas.UriTemplate.<init>(UriTemplate.java:56)
at org.springframework.hateoas.Link.<init>(Link.java:94)
at org.springframework.hateoas.hal.Jackson2HalModule$HalLinkListDeserializer.deserialize(Jackson2HalModule.java:583)
at org.springframework.hateoas.hal.Jackson2HalModule$HalLinkListDeserializer.deserialize(Jackson2HalModule.java:528)
at com.fasterxml.jackson.databind.deser.impl.FieldProperty.deserializeAndSet(FieldProperty.java:136)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:369)
I tried serving this JSON locally so I can remove the next and prev links from it, and it works fine. I had a look in the code and it fails because it's trying to do new UriTemplate(href) which does not accept null as a parameter.
We are running Spring Boot version 2.1.8.RELEASE and spring-hateoas version 0.25.2. Below is the code that makes the request:
ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
mapper.configure(MapperFeature.DEFAULT_VIEW_INCLUSION, false);
Jackson2HalModule jackson2HalModule = new Jackson2HalModule();
mapper.registerModule(jackson2HalModule);
MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
converter.setSupportedMediaTypes(MediaType.parseMediaTypes(Arrays.asList("application/json;charset=UTF-8", "application/hal+json")));
converter.setObjectMapper(mapper);
RestTemplateBuilder restTemplateBuilder = new RestTemplateBuilder();
RestTemplate restTemplate = restTemplateBuilder.messageConverters(Collections.singletonList(converter)).build();
HttpHeaders headers = new HttpHeaders();
headers.set("Accept", "application/json, application/*+json");
headers.setContentType(MediaType.APPLICATION_JSON_UTF8);
HttpEntity<String> entity = new HttpEntity(entity.toString(), headers);
ParameterizedTypeReference<Resources<Panden>> typeRefDevices = new ParameterizedTypeReference<>() {};
ResponseEntity<Resources<Panden>> result = restTemplate.exchange(endpoint, HttpMethod.POST, entity, typeRefDevices);;
After spending some time looking through the code of Jackson2HalModule I found a solution for this problem. There is no configuration that I can use to tell the library to simply ignore links that have null values. What I can do instead is override the deserialize() method so that I can add a null check and avoid the problem.
In order to do this firs I had to create the new link deserializer class. It needs to extend Jackson2HalModule.HalLinkListDeserializer then I just copy & pasted the code from the library source with the modifications I wanted.
public class CustomHalLinkListDeserializer extends Jackson2HalModule.HalLinkListDeserializer {
@Override
public List<Link> deserialize(JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException {
List<Link> result = new ArrayList<Link>();
String relation;
// links is an object, so we parse till we find its end.
while (!JsonToken.END_OBJECT.equals(jp.nextToken())) {
if (!JsonToken.FIELD_NAME.equals(jp.getCurrentToken())) {
throw new JsonParseException(jp,
"Expected relation name", jp.getCurrentLocation());
}
// save the relation in case the link does not contain it
relation = jp.getText();
if (JsonToken.START_ARRAY.equals(jp.nextToken())) {
while (!JsonToken.END_ARRAY.equals(jp.nextToken())) {
CustomLink link = jp.readValueAs(CustomLink.class);
String href = link.getHref() != null ? link.getHref() : "http://dummy";
result.add(new Link(href, relation));
}
} else {
CustomLink link = jp.readValueAs(CustomLink.class);
String href = link.getHref() != null ? link.getHref() : "http://dummy";
result.add(new Link(href, relation));
}
}
return result;
}
}
Then, in order to tell the Jackson2HalModule to use the new class I had to create my own MixInAnnotation class where I specify that links are deserialized using CustomHalLinkListDeserializer
public abstract class CustomResourceSupportMixin extends ResourceSupport {
@Override
@XmlElement(name = "link")
@JsonProperty("_links")
@JsonInclude(JsonInclude.Include.NON_EMPTY)
@JsonSerialize(using = Jackson2HalModule.HalLinkListSerializer.class)
@JsonDeserialize(using = CustomHalLinkListDeserializer.class)
public abstract List<Link> getLinks();
}
The new MixInAnnotation can be set while configuring jackson2HalModule as below
ObjectMapper mapper = new ObjectMapper();
Jackson2HalModule jackson2HalModule = new Jackson2HalModule();
jackson2HalModule.setMixInAnnotation(ResourceSupport.class,
CustomResourceSupportMixin.class);
mapper.registerModule(jackson2HalModule);